An arrangement according to the first aspect of the invention is known. In the known arrangement, the mounting element which is bonded to the tube wall is made of a soft nickel-iron alloy, so as to have a similar coefficient to expansion to that of the tube. The material of the mounting element connected to the frame matches that of the mounting element bonded to the tube wall, with the intention of alleviating undesirable wear of the spigot. However, a disadvantage arises in that, since the mounting elements are both relatively soft, wear is still a problem and the arrangement is not very rigid.
An arrangement to the second aspect of the invention is also known, in which the spigot projects from the tube wall in order to be able to engage with the socket.